The present invention relates to a system for driving a reactive load.
For simplicity of description, reference will be made below to applications in which the reactive load is capacitive but it is intended that the invention may, in practice, also be implemented in just the same manner in applications in which the reactive load is inductive, bearing in mind the equivalence of the voltage and current behavior of capacitances and inductances.
In order to supply energy to a load in a controlled manner, be it a capacitive, an inductive, or a resistive load, it is well known to use an amplifier supplied by a direct-current voltage supply and controlled so as to modulate the supply of a variable quantity of energy to the load in predetermined manner, that is, so as to achieve a given current or voltage waveform in the load.
An application of this type with a capacitive load is shown in FIG. 1 of the appended drawings. An amplifier 1 has an output stage represented schematically by two controllable current sources G1, G2, connected in series between the rails of a voltage supply, indicated Vs and by the earth symbol. The output terminal of the amplifier, which is the connection node between the two current sources is connected to a capacitive load represented by a capacitor Cl. A control circuit 2 supplies control signals to the amplifier so as to modulate the supply or absorption of current by the current sources G1 and G2, and hence the supply to the load Cl, in accordance with a predetermined program.
It is assumed that current is supplied to the load Cl so as to achieve therein a voltage waveform as shown in FIG. 2, that is, that the capacitor Cl is to be charged from 0 to a voltage V1, starting from a time t0, in a period t0-t1, the charge in the capacitor Cl is constant in a period t1-t2, and that it is to be discharged in a period t2-t3. After a delay period t3-t4, the capacitor Cl is again charged in period t4-t5. The control circuit 2 will therefore activate the current source G1 from the time t0 to the time t1 with the current source G2 deactivated, deactivate the current source G1 in period t1-t2, activate the current source G2 in period t2-t3 with the current source G1 deactivated, deactivate the current source G2 in period t3-t4, and reactivate the current source G1 in period t4-t5.
A graph of the current I in the load Cl as a function of time is shown in FIG. 3 and a graph of the power Pd dissipated in the current sources G1 and G2 as functions of time is shown in FIG. 4. It can easily be shown that, in a practical embodiment, if Cl=2 xcexcF, t0-t1=6 xcexcs, t1-t2=4 xcexcs, Vl=35V and Vs=40V, there is a constant charge current I1=11.6 A, a constant discharge current I2=17.5 A, an instantaneous maximum power P1=464 W dissipated in the current source G1, an instantaneous maximum power P2=612.5 W dissipated in the current source G2, a mean power in the period t0-t1 of 156.6 W, a mean power in the period t2-t3 of 122.5 W, and a total power dissipated in the period t0-t3 of 272.1 W. The areas A1, A2, and A3 represent the power dissipated by the error amplifier 1.
In many applications, the power dissipation of the system described above is considered excessive. A need has therefore arisen for alternative systems for driving a reactive load with lower power dissipation.
According to an embodiment of the present invention a system for driving a reactive load is presented. The system includes means for controlling a load by supplying variable quantities of energies to the load in a predetermined manner as well as a reactive component coupled to the load through an electronic switch that forms a resonant circuit with the load when the switch is closed. The system also includes means for activating the switch and a control unit coupled to both the supply means and the activation means to coordinate their operation to supply energy to the load in a predetermined manner.